(1) Field of the Invention
The present invention relates to a dielectric ceramic composition comprising oxides of strontium, zinc, niobium, and titanium, which is suitable as a dielectric resonator or resonator material. More particularly, the present invention relates to a dielectric ceramic composition which shows a large no-load value Q (Qu) and a specific inductive capacity (.epsilon..sub.r) and has an excellent and stable temperature coefficient (.tau..sub.f) of the resonance frequency when used for a dielectric resonator to be used in a microwave frequency band, especially a frequency band of 2 to 6 GHz.
(2) Description of the Related Art
When a cavity resonator is used for a microwave circuit, the size of the resonator becomes almost equal to the wavelength of the microwave. Accordingly, in order to diminish the size of the circuit, there has been adopted a method in which a dielectric ceramic is used for a resonator and the wavelength per se is shortened, thereby reducing the size of the circuit.
The size of the dielectric resonator is in inverse proportion to .sqroot..epsilon..sub.r and, as the dielectric constant is large, the circuit can be diminished. Accordingly, a ceramic composition having a larger specific inductive capacity is generally preferred as a resonator or resonator material. However, if the specific inductive capacity is extremely large, the resonator is overdiminished and handling becomes difficult. Therefore, the required value of the specific inductive capacity differs according to the frequency band in which the resonator is used.
One characteristic required for a dielectric resonator is a small dielectric loss. In other words, the no-load value Qu should be as large as possible and the temperature characteristic should be stable. However, in a dielectric resonator having a large specific inductive capacity, the no-load value Qu is usually small, and the no-load value Qu is further reduced as the frequency becomes high.
Various ceramic compositions such as those of TiO.sub.2 -ZrO.sub.2 -SnO.sub.2, CaTiO.sub.3 -MgTiO.sub.3 -LaO.sub.3.2TiO.sub.2, and Ba(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 systems have been proposed as the ceramic composition for dielectric resonators. However, these ceramic compositions are defective in that when a dielectric resonator to be used in a frequency band of 2 to 6 GHz is fabricated, since the specific inductivity capacity is small, the size of the resonator is considerably large in a frequency band of 2 to 6 GHz, though the size of resonator may be reduced in a frequency band of, for example, about 11 GHz.
Japanese Examined Patent Publication (Kokoku) No. 50-34759 discloses a dielectric ceramic composition suitable as a ceramic battery material, which is represented by the following composition formula, EQU Sr[Ti.sub.1-x (Zn.sub.1/3 Nb.sub.2/3).sub.x]O.sub.3
wherein x is a number of from 0.02 to 0.08.
According to the disclosures of this patent publication concerning the electrical characteristics of this ceramic composition, the tan .delta. value is small and about 0.03% (the no-load value Qu is about 3300) as measured at a low frequency such as 1 KHz. Accordingly, in a high frequency band of 2 to 6 GHz, the dielectric loss is too large and the ceramic composition is not practical for use as a dielectric resonator or resonator material. Furthermore, even if a dielectric resonator is fabricated by using this composition, the temperature coefficient of the resonance frequency becomes too large.